1. Field of the Invention
The present invention relates to a film formation method by which a film is formed over a substrate. Further, the present invention relates to a method for manufacturing a light-emitting device in which a layer including an organic compound which is manufactured by the film formation method is used as a light-emitting layer.
2. Description of the Related Art
In recent years, a technique relating to a flat panel display has been remarkably progressed. As a typical flat panel display, a liquid crystal display device and a light-emitting device can be given.
A liquid crystal display device operates by control of transmittance of light, which is emitted from a backlight, at each pixel by liquid crystal elements arranged in matrix. A light-emitting device operates by control of a lighting state or a non-lighting state of self-light-emitting elements arranged in matrix at each pixel. As compared to a liquid crystal display device, a light-emitting device has many merits. Since a light-emitting element provided in a light-emitting device emits light by itself, visibility is high and backlight is not necessary, which is suitable for thinning, and a viewing angle is not limited. Further, since utilization efficiency of light is high, a light-emitting element consumes low power, and response speed is high. Accordingly, a light-emitting device using a light-emitting element has attracted attention as an alternative to a display device or a liquid crystal display device which uses a cathode-ray tube. A light-emitting device having such merits is mounted on an electronic device such as a mobile phone or a digital still camera, and has been gradually put into practical use.
Light-emitting devices can be roughly classified into passive-matrix (simple-matrix) light-emitting devices and active-matrix light-emitting devices. Since a supply of current to a light-emitting element can be maintained to some extent even after a video signal is input, active-matrix light-emitting devices can be flexibly adapted to enlargement and high definition of a panel, and are becoming the mainstream. Pixels have various structures in active-matrix light-emitting devices. Each of the structures is distinctively and technically devised, in which at least a thin film transistor (hereinafter referred to as a TFT) which controls input of a video signal to a light-emitting element and a TFT for applying current to the light-emitting element are provided in each pixel.
It is considered that a light-emitting element has a mechanism in which, when voltage is applied between a pair of electrodes which interpose a layer including an organic compound, electrons injected from a cathode and holes injected from an anode are recombined in an emission center of the layer including an organic compound to form a molecular exciton, and energy is released to emit light when the molecular exciton returns to the ground state. As excited states, a singlet excited state and a triplet excited state are known, and light emission is considered to be possible through either of these excited states.
The layer including an organic compound typically has a stacked structure of “a hole transporting layer, a light-emitting layer, and an electron transporting layer”. Further, EL (electro luminescence) materials which form the layer including an organic compound are roughly classified into low molecular (monomer) materials and high molecular (polymer) materials. Films of high molecular materials are mainly formed by a spin coating method, a printing method, an ink-jet method, or the like which is a wet film formation method, whereas films of low molecular materials are mainly formed by a vacuum evaporation method which is a dry film formation method. Since low molecular materials are easily purified, impurities are not easily mixed. Therefore, when low molecular EL materials are used, a highly-reliable light-emitting device with long life can be manufactured.
However, when films of low molecular materials are formed by a vacuum evaporation method, the following become obstacles to reduction of the manufacturing cost of a light-emitting device: utilization efficiency of EL materials is about 1% or less, which is extremely low, and EL materials are extremely expensive. Thus, a technique which improves utilization efficiency of low molecular materials has attracted enormous attention. As such a technique, Reference 1 (Changhun Chriss Hwang, Plane Source and In-line Deposition System for OLED Manufacturing, SID 06 DIGEST, pp. 1567-1570) can be given, for example.